using System;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

    public static IEnumerable<string> Y()

    {

        var list = new List<string> {"1","2","3","4","5"};

        foreach(var i in list)

        {

            yield return i;

        }

    }

    public static void Main()

    {

        var ys = Y();

        Console.WriteLine("first ys");

        Console.WriteLine(string.Join(",", ys));

        Console.WriteLine("second ys");

        Console.WriteLine(string.Join(",", ys));

    }

}